The present invention relates to a method and system for controlling permanent magnet motors. More specifically, the present invention relates to a method and system for controlling permanent magnet motors utilizing a three region control strategy.
Drive systems for electric vehicles seek to provide high peak power for passing and acceleration, and high efficiency at light loads and highway cruising to provide acceptable range, all at a reasonable cost. Permanent magnet (PM) motors (drives) have been considered for electric and hybrid vehicle drive systems due to their high performance and efficiency, flexibility, and low noise and torque ripple. In a permanent magnet motor, a rotary field is established by a set of permanent magnets secured to the rotor and a second rotary field is established by sequentially energizing a set of phase windings secured on the stator. Motor torque results when these fields are rotating synchronously and out of phase. Torque is maximum when these fields are in quadrature and zero when they are coincident (in phase). A position or emf sensing device detects the position of the rotor and a logic circuit energizes the stator windings relative to the detected position of the rotor to accelerate the rotor. The motor phase windings are energized as dictated by a control strategy to control the motor. Typically, in order for vehicles powered by PM motors to attain highway speeds, the PM motors have a reduced number of turns. This follows as the induced voltage in the PM motor is a linear function of speed. Other approaches to increase (or extend) the speed range have been to increase the supply voltage to the PM motor or to utilize the PM motor reactance to drop the excess voltage.
It is, therefore, a primary object of the present invention to provide a method for efficiently controlling permanent magnet motors in electric and hybrid vehicles. It is further an object of the invention to maximize drive efficiency during the Federal Testing Procedures (FTP) driving cycle where the drive operation is primarily at low speeds and light loads, provide necessary peak power for acceleration without demagnetizing the rotor due to eddy current heating, and provide for field weakening to extend the speed range to reduce the inverter cost.
In accordance with these and other objects the present invention utilizes a three region control strategy. In a first region, the permanent magnet motor (i.e., a permanent magnet brushless d.c. motor having a trapezoidal distribution of magnet field in its airgap) is operated at, a 120xc2x0 conduction rectangular wave mode at reduced phase currents, and below the no-load speed. The motor phase current commutation causes eddy current losses in the rotor magnets and core which are insignificant due to the low phase currents and relatively low rotor speed. Meanwhile, the inverter switching losses are kept low as two switches are in use (on/off) for each current commutation during the 120xc2x0 conduction mode. In a second region, the permanent magnet (PM) motor is operated at a 180xc2x0 conduction sinusoidal wave mode with high phase currents. The 180xc2x0 conduction sinusoidal wave mode minimizes the commutation loss in the rotor. In a third region, the PM motor is operated above its no-load speed or in a field weakening mode. At these higher speeds the slot ripple and commutation losses on the rotor increase, and the demagnetizing component of the armature reaction increases due to field weakening. Commutation losses are minimized (or eliminated) through sinusoidal current operation. In the field weakening mode, the phase current conduction angle is set to 180xc2x0 and the phase currents become sinusoidal.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.